<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8" />
		<title></title>
	</head>
	<body>
		<button id="btn1">1</button>
		<button class="btn2">2</button>
		<button name="btn3">3</button>
		<button>4</button>
		<button>5</button>
		<script type="text/javascript">
			var btn1 =  document.getElementById("btn1");
			console.log(btn1);
			console.log(btn1.id);
			console.log(btn1.innerHTML);
			btn1.innerHTML = "<b>123</b>"
			/************************/
			var btn2 = document.getElementsByClassName("btn2")[0];
			console.log(btn2);
			/***********************/
			var btn3 = document.getElementsByName("btn3")[0];
			console.log(btn3);
			var btn4 = document.getElementsByTagName("button")[3];
			console.log(btn4);
			
			var btn5 = document.querySelector("#btn1+button+button+button+button");
			console.log(btn5);
			
			btn5 = document.querySelectorAll("button")[4];
			console.log(btn5);
		</script>
	</body>
</html>
